But here is a more serious answer:
Than Thain, from Tidal Gardens, has many YouTube videos on coral propagation. He has one where he cuts a toadstool and although it is much smaller than what you have, his method might be of interest to you. He removes the colony from the aquarium and sets it aside in a different container filled with tank water. He will cut the piece and put it back in this same container and let it sit in the container for while in which the toadstool is releasing its toxins. He also adds a few drops of iodine to the container to aid in the healing process. When he feels the toxin releasing phase is over (which I really dont know of a proven way to test when this is actually over) he will put it back into his display. If I remember correctly, he shows a clip of what the coral looks like the next day and it, along with coral around it, look fine. It is a great channel to subscribe to. Maybe someone else will chime in here as well.